The invention generally relates to locking mechanisms for power driven shafts More particularly, the invention relates to an automatic shaft lock for hand-held power tools.
Mechanisms of the general type under consideration were previously used as safety-lock couplings on conveyor drives and boat steering mechanisms and also as locks for power-driven tools. It is desirable to provide locking capability for hand-held power tools so that they can be used manually. For example, if a power tool lacks sufficient torque to tighten screws fully, it is advantageous to be able to use the same tool to tighten them manually Also, it is often desirable to set a screw manually in order to control the final torque applied to the screw. However, the typical tool has an implement-carrying unit connected in line with the motor and drive mechanism which, when torque is applied to the implement, results in slippage through the motor. Thus, it would be useful to lock the shaft of a hand-held power tool for manual operation.
In the past, some shaft locks have been incorporated into hand-held power tools to permit manual operation. Typically, the locks require the operator to activate a knob or button to lock the implement carrying unit for manual operation. Of interest are U.S. Pat. Nos. 4,448,098 and 3,802,518 which disclose mechanisms requiring the operator to set or actuate the locking mechanism. However, when the operator actuates the locking mechanism, power operation is restricted until the lock is de-activated.
Other tools feature an automatic locking device which locks the implement carrying unit against rotational movement relative to the tool housing unless the power is activated. One such device is the AEG Model EZ 502 made by Matsushita of Japan. This device uses a pair of small pawls, each resting in and pivoting about cavities formed in the housing, to engage teeth on the inside of a ring gear, thereby locking the carrying unit against rotational movement relative to the tool. When the motor is energized, a cam on the ring gear lifts a pawl out of engagement, allowing corresponding rotation of the implement end by the motor.
There are several disadvantages to the above device. First, the pawls are hard to manufacture because of their small size. Second, because the components are small, the device is difficult to assemble. Third, the engaging surfaces of the pawl are correspondingly small resulting in rapid wear and, potentially, a short lifetime. And fourth, use of a ratchet and pawl may produce an irritating clicking noise and undesirable friction resulting in reduction in power and unnecessary generation of heat.
U.S. Pat. No. 3,243,023 discloses a shaft locking mechanism without a pawl or ratchet that has been used on vehicle steering mechanisms to eliminate feedback of energy through the shaft. The '023 device utilizes a coupled output shaft and input shaft with multiple pairs of cylindrical rollers, each pair separated by a compression spring to maintain the rollers in locking positions respectively between a circular housing and a curved cam on the output shaft. When torque is applied to the input shaft, fingers located on the shaft contact the rollers compressing the springs and urging the rollers out of locking positions. Such a shaft locking mechanism also has been used for rudder-control and material handling equipment such as belt conveyors (See Tuttle, Stanley B., Mechanisms For Engineering, John Wiley & Sons, Inc. 1967).
The '023 device relies on a spring to force apart each pair of rollers in order to accomplish wedging. The torque applied to the input shaft must work against this spring force to unlock the device. Also, if the spring force should fail, due to repeated compression, for example, the locking action would fail. The device requires a pair of rollers separated by a spring to accomplish locking in both directions.
Therefore, it is an object of the invention to provide a locking drive shaft not requiring manual actuation of a locking mechanism by the operator, without the difficulty of manufacture and assembly of prior devices and with less possibility of mechanical failure.
It is another object of the invention to provide an automatic shaft lock for hand tools without restricting power operation and without the difficulty of manufacture and assembly of prior devices.
It is yet another object to provide an embodiment of an automatic locking device that does not produce a clicking sound or generate unnecessary friction during application of power.
These and other objects, features and advantages will become apparent from the following discussion of a preferred embodiment and an alternative embodiment of the invention.